The present invention relates to gravity meters which detect changes in ambient gravitational fields as they are moved from one location to another or as the field varies with time. Particularly, the present invention is adapted to be utilized in measuring changes in the gravitational field surrounding a bore hole, for example, in the exploration for oil or other materials.
Geophysicists in particular have long been interested in the bulk measurement of rock densities in situ and one of the first measurements of this type utilized pendulum bases situated in vertical mine shafts. As more portable gravity meters became available, geophysicists recognized their potential in evaluating subsurface anomalies; extensive underground surveys were completed in the 1940's and 1950's which provided impetus for the development of a down hole gravity meter.
One of the early solutions to the problem of measuring changes in ambient gravitational fields was described in U.S. Pat. No. 2,607,487 issued to Worden, which utilized a weight or reaction mass disposed at one end of a pivotal horizonal arm. In the 1960's, several companies developed vibrating string gravity meters which were capable of accuracies of 0.1 milligals, but such devices only stimulated the desire for greater accuracy.
A major breakthrough in gravity meters designed for use in down hole or well bore applications came when LaCoste and Romberg developed a geodetic meter to operate inside a down hole sonde. In the mid 1970's, this meter was reduced in size such that it would be capable of surveying a 51/2" casing while at the same time being capable of surveying a well bore having a deviation from the vertical as much as 14 degrees.
Typically, gravity meters of the type known utilize a weight disposed of one end of a horizontal weight arm with the other end of the arm being secured to a pivotal support. This weight arm is maintained at a desired reference position by a mainspring which is often secured to the end of a second support arm mounted to the frame of the instrument. The mainspring is selected to counteract the force of gravity acting on the weight arm over a specific range of gravity, and a mechanical adjustment system is attached to the support arm to balance the weight arm to a desired reference point. Once the instrument is nulled, a change in the ambient gravitational field between two observation stations or over time causes a displacement of the weight arm. Measurement of this displacement may be utilized to calculate the change in the gravitational field. Accordingly, the device operates on the principle of balancing the force of gravity by varying the force applied by the spring.
Notwithstanding these past developments, many problems have confronted both those who produced these gravity meters and those who utilize them in the field. For example, one difficulty stems from the need for a gravity meter which is both relatively small in size so that it may be operated internally of a well bore while at the same time being highly accurate to detect very slight changes in its ambient gravitational field. As well bores have gotten deeper, increased temperatures have affected the application of these meters.
Yet another problem confronts the use of these gravity meters which problem arises as a result of the world-wide variance in gravitational field. As noted, each of the devices operates within a range determined by the characteristics of its mainspring. The problem is that no mainspring has yet been capable of accurately responding to the full range of gravity variance encountered throughout the world while at the same time having suitable sensitivity for bore hole gravity measurements so that it thus becomes necessary to select a meter for use in each local area according to the gravity field range in that area. Users of these meters must therefore have several different meters to cover the complete range of gravity variance.
Another problem confronting these devices is the need for a readout system which will accurately reflect movement of the reaction mass so that the changes in the ambient gravitational field of the zone can be calculated from the magnitude of that movement. For example, in U.S. Pat. No. 3,245,263, issued to Cornelison, an optical readout system is disclosed. Generally, these optical readout systems, while functional, are susceptible to being dislocated or otherwise damaged while the gravity meter is in the well bore which can cause futher delays and expenses in measuring the gravitational field. LaCoste and Romberg developed a readout system based on a metal weight arm positioned between two conductor plates. A square wave signal is placed on each plate with the signals normally being 180 degrees out of phase. The position of the weight arm can be thus affected by a direct current signal placed on a selected one of the plates while the position of the weight arm is determinable from monitoring the resultant signal generated on the weight arm from the two square wave signals.